This invention relates to an anti-lock control system for vehicle wheel brakes.
When the brakes of a vehicle are applied, a braking force between the wheel and the road surface is generated that is dependent upon various parameters including the road surface condition and the amount of slip between the wheel and the road surface. For a given road surface, the force between the wheel and the road surface increases with increasing slip values to a peak force occurring at a critical wheel slip value. As the value of wheel slip increases beyond the critical slip value, the force between the wheel and the road surface decreases. Stable braking results when the slip value is equal to or less than the critical slip value. However, when the slip value becomes greater than the critical slip value, braking becomes unstable resulting in sudden wheel lockup, reduced vehicle stopping distance and a deterioration in the lateral stability of the vehicle.
Numerous wheel lock control systems have been proposed to prevent the wheels from locking up while being braked. These systems generally prevent a wheel from locking by controlling the applied brake pressure when an incipient wheel lockup condition is sensed so as to maintain substantially the maximum possible braking force between the tire and road surface while at the same time preventing the wheel from operating in the unstable braking region.
Some of the known wheel lock control systems utilize an "independent" mode of braking wherein each of the front and rear vehicle wheels are controlled independently so as to establish the maximum possible braking force at each wheel during wheel lock controlled braking. By so maximizing the braking forces at each wheel, the stopping distance of the vehicle is minimized. However, under certain conditions, this mode of operation can lead to reduced vehicle stability. The conditions for this occurrence are either grossly different coefficients of friction between right and left sides of the vehicle, or severe steering maneuvers while braking.
Other forms of known wheel lock control systems utilize what is referred to as the "select low" mode of wheel lock controlled braking which provides for improved vehicle stability and steerability when the vehicle is being braked on a split coefficient of friction surface. In this form of wheel lock control system, the front brakes are typically controlled independently as above described but the rear wheel brakes are controlled such that the brakes of the rear wheel being braked on a high coefficient of friction surface is regulated in response to the conditions of the rear wheel being braked on a lower coefficient of friction surface. While this mode of braking increases the vehicle stopping distance, vehicle stability and steerability is improved. For example the lateral friction force of the rear wheel being braked on the high coefficient of friction surface is increased since it is being controlled to a low slip value.